Outcrossing between johnsongrass, sorghum studied

COLLEGE STATION - Johnsongrass and sorghum might be considered 'kissing kin,' but a Texas A&M AgriLife Research team wants to know if there is more going on in the grain sorghum production fields and bar ditches of South and Central Texas than meets the eye.

An experimental field in College Station where outcrossings between sorghum and johnsongrass are studied by the Texas A&M AgriLife Research team. (Texas A&M AgriLife photo by Dr. Muthu Bagavathiannan)

Dr. Muthu Bagavathiannan, weed scientist; Dr. Bill Rooney, sorghum breeder; and Dr. Patricia Klein, sorghum geneticist and molecular biologist, all with AgriLife Research in College Station, have teamed up to study gene flow between sorghum and johnsongrass.

They have secured funding from a U.S. Department of Agriculture National Institute of Food and Agriculture Biotechnology Risk Assessment Grant, award No. 2017-33522-27030, for the study.

'The two plants are genetically related and the fear is that through gene flow, johnsongrass can pick up traits from sorghum,' Bagavathiannan said. 'What we don't know is the frequency at which this happens, the genetic determinants of outcrossing and what the hybrid progeny look like in terms of fitness and invasiveness.'

While they started this type of work in 2014, he said, the funding last year allowed expansion of the research questions.

Rooney and his associate scientist Dr. George Hodnett will look at the cytogenetics - how chromosomes relate to cell behavior - of outcrossing between these two species and how they are influenced by sorghum genotype.

Klein will investigate, using the genotyping by sequencing approach, the chromosomal location of potential genetic barriers affecting successful hybridization between the two.

This four-year project - 2017-2021 - also includes work by postdoctoral research assistant Dr. Sara Ohadi and master's student Cynthia Sias, both working with Bagavathiannan.

'Earlier studies have looked at the gene flow from sorghum to johnsongrass, but what also is important is to see gene flow in the opposite direction,' Rooney said.

The F2 progeny - the offspring of a cross between any two unrelated seed lines - of sorghum hybrids segregate for male sterility and when present near johnsongrass, there is a high likelihood the male sterile sorghum will receive pollen from johnsongrass, he said.

'We conducted a detailed survey to document the presence of feral sorghum and johnsongrass on South Texas roadsides, and we will determine if there are hybrids occurring in nature, what they look like and if they are more aggressive than others,' Bagavathiannan said.

He said they also are conducting field-level gene flow studies involving different genetic backgrounds and will grow harvested hybrid seed to observe growth characteristics and fitness.

Historically, controlling grass weeds in grain sorghum has been difficult because sorghum is also a grass; there have been very limited over-the-top herbicide options available for grass control in sorghum.

However, acetolactate synthase or ALS-inhibitor herbicide tolerance has been developed in sorghum over the past few years under the name Inzen sorghum that will allow for effective post-emergence grass control.

'This is a trait that has already been approved for commercial cultivation and growers are currently waiting on hybrid seed availability,' Bagavathiannan said. 'But the longevity of this technology depends on how well we confine, mitigate and/or manage gene flow between the two species.

'Because johnsongrass is a problematic weed in sorghum fields, transfer of the herbicide resistance trait into johnsongrass will make the technology ineffective,' he said. 'This presents a serious agronomic problem. It doesn't matter whether the resistance was introduced through classical breeding or through transgenic means.'

Gene flow between the two species can also cause ecological and environmental problems, especially if the introduced trait provides an adaptive advantage to the progeny such as disease or insect resistance, Bagavathiannan said. In such a scenario, gene flow can be a boost for the weed. However, gene flow is not usually a concern with non-adaptive traits such as grain biochemical quality.

'Findings of this study will guide the development of suitable strategies to address this problem and help the sorghum industry in sustaining the utility of upcoming technologies,' he said.